1. Field of the Invention
This invention relates generally to wear and impact resistant bodies for use in cutting, machining, drilling and like operations, as well as for use as wear surfaces such as lapping stops, valve seats, nozzles, etc. More particularly, the invention relates to such bodies which comprise polycrystalline diamond and cemented metal carbide pressed at ultra high pressure and temperature.
As used in the following disclosure and claims, the term "polycrystalline diamond" along with its abbreviation "PCD" is intended to refer to the type of material which is made by subjecting individual diamond crystals to ultra high pressure and temperature such that intercrystalline bonding occurs. Generally, a catalyst/binder material is used to ensure adequate intercrystalline bonding. This material is also often referred to in the art as "sintered diamond".
Also, in the following disclosure and claims the term "precemented carbide" is intended to refer to the type of material resulting when grains of a carbide of one of the group IVB, VB, or VIB metals are pressed and heated (most often in the presence of a binder such as cobalt, nickel, or iron as well as various alloys thereof) to produce solid, integral pieces. The most common and readily available form of precemented carbide is tungsten carbide containing a cobalt binder.
2. Prior Art
In several applications, polycrystalline diamond has displayed particular advantages over single crystal diamond. In particular, PCD is more impact resistant than single crystal diamond. Single crystal diamond has relatively low impact resistance, due to its extremely high modulus of elasticity, as well as its specific planes of cleavage in which relatively low forces can cause fracturing of the crystal. PCD, on the other hand, which is made up of randomly oriented individual crystals, alleviates problems caused by the planes of cleavage in the single crystal form. However, PCD is still relatively low in impact resistance because of the high modulus of elasticity of diamond. This low impact resistance is a problem because, in many applications, PCD "wears" not from erosion of atomic layers, but rather from fracturing and spalling occurring at both macro and microscopic scales.
The relative brittleness of PCD was recognized early, and as a result the first commercially available PCD products included a metallic backing layer or substrate bonded directly to the diamond layer, as shown in U.S. Pat. No. 3,745,623. The most common form of this "composite compact" to date has been a planar disc of PCD sintered directly onto a precemented disc of tungsten carbide by means of a high pressure high temperature press cycle.
This arrangement, in which the PCD is supported by a single precemented carbide mass or similar substrate, has also provided advantages for the attachment of PCD. Diamond is relatively inert. As a result, it is difficult if not impossible to attach PCD to a tool support or other surface through conventional brazing techniques. Accordingly, providing PCD with a metallic backing which can itself be brazed provides a suitable means for brazing the PCD composite compact to a tool support.
Unfortunately, certain problems are found in the composite compacts produced as above, i.e. with a layer of PCD directly attached to a single planar substrate. One of these problems has been the limitation on the design of polycrystalline diamond tools to those configurations in which the diamond layer can be adequately supported by the carbide substrate. Although some work has been done to expand the possibilities (see for example U.S. Pat. No. 4,215,999 where a cylinder of polycrystalline diamond is sintered around a core of precemented carbide), there are conceivable uses for PCD in tools which are difficult or impossible to implement with the conventional composite compact. For example, rotary tools such as miniature grinding wheels and drills which need to be symmetrical about a line and in which the working faces are subject to tangential forces have not been commercially implemented.
Another problem arises because the precemented carbide substrate has a higher coefficient of thermal expansion than that of the PCD. Because the bond between the diamond layer and the precemented carbide substrate is formed when both materials are at a temperature in the range of 1,300.degree. to 2,000.degree. C., stresses are created when the composite compact cools and the carbide substrate shrinks more than the diamond. Because the diamond layer is less elastic than the carbide substrate, these stresses often cause cracking in the diamond layer, either during the cooling phase, during brazing, or during use of the composite compact.
Another limitation on the use of substrates for supporting or attaching PCD compacts is the requirement that the composition of the substrate be chemically compatible. In particular, it is important that the substrate material not be detrimentally reactive toward the diamond or the catalyst/binder material. For example, it has been difficult if not impossible to sinter PCD on a steel or other ferrous alloy substrate because of the strong tendency for the iron to dissolve or catalyze the graphitization of the diamond. This is unfortunate in that steel would otherwise be a good substrate material as it is easier to work with than cemented carbides. Steel also has a lower modulus of elasticity and would therefore be preferred in some applications such as rock bits and the like where high impact forces are encountered. Steel substrates would also be preferable in that they are easier to weld to and easier to install in a tool with a simple interference fit.
Furthermore, when a precemented carbide mass is relied on to increase the impact resistance of PCD, the diamond layer is preferably relatively thin so that the diamond is never too far from its support. This restriction on the thickness of the diamond layer naturally limits both the life expectancy of the composite compact in use as well as the designs for PCD diamond tools.
Yet another problem which has limited the thickness of the diamond layer in composite compacts is caused by the problem of "bridging". Bridging refers to the phenomenon that occurs when a fine powder is pressed from multiple directions. It is observed that the individual particles in a powder being pressed tend to stack up and form arches or "bridges" that the full amount of pressure often does not reach the center of the powder being pressed. The inventors have observed that when a 1 micron diamond powder is used to make a PCD compact which is more than about 0.06 inches thick, the PCD toward the center of the piece is usually not as well formed as the exterior portions of the compact. This condition can result in cracking and chipping of the diamond layer.
In the co-pending application Ser. No. 600,399, by David R. Hall, an improved PCD composite material is described which has partially alleviated some of the above-mentioned problems. In general, the material disclosed in that application comprises a mixture of diamond crystals and precemented carbide pieces which is pressed under sufficient heat and pressure to form a polycrystalline diamond matrix with cemented carbide dispersed therein, or alternatively a cemented carbide matrix with polycrystalline diamond dispersed therein. This composite PCD/cemented carbide material was found by the one of the inventors to have increased toughness over standard PCD, thus making it attractive for high impact uses such as earth boring, cement sawing, and the like.
Also, the addition of the precemented carbide pieces to the PCD was found beneficial to the properties of composite compacts with a cemented carbide backing. In particular, the stresses at the interface between the PCD layer and the backing caused by the differing coefficients of thermal expansion are reduced because the presence of the cemented carbide pieces dispersed within the PCD layer tends to make the thermal expansion properties of the PCD layer more like that of the backing.
In addition, the inclusion of the pieces of cemented carbide to the PCD was found to lessen the problems caused by bridging. In particular, the precemented pieces of carbide did not compress appreciably and thereby improved the pressure distribution in the pressing cell. Accordingly, the new composite PCD material could be better pressed in thicker pieces.
Although at certain low concentrations of dispersed cemented carbide the wear resistance of this composite material was surprisingly higher than standard PCD, generally the wear resistance was less than that of standard PCD. As could be expected, the higher concentrations of cemented carbide possess lower wear resistance. In many applications, this compromise of the wear resistance of the PCD body in order to achieve increased toughness is acceptable. However, it would certainly be desirable to have the optimum of both wear resistance and impact resistance in each compact. Also, in certain wear part applications, such as PCD bearings, it is important that the surface of the PCD part be homogeneous so that the surface can wear at a uniform rate.